add KHR extensions to OpenCL registry

[bashbaug]

See internal issue 188.

Updates scripts and the extension registry to include KHR extensions (mostly new) in addition to EXT and vendor-specific extensions (existing).

Summary of the changes are:

1. I modified the script to generate separate unnumbered, alphabetized lists for KHR extensions and non-KHR extensions instead of a single ordered list based on the extension number for (primarily non-KHR) extensions. This was the biggest change (but not too bad, the script is very short).
2. I modified the registry webpage to include both extension lists instead of only the primarily-non-KHR extension list.
3. I added all KHR extensions to registry.py with links into the extension spec.
4. A few minor changes to Makefiles or other supporting scripts.

Needs working group sign-off before merging.

[alycm]

LGTM, though I'm not setup to view the PHP locally.

[bashbaug]

Merging as we discussed in the October 5th teleconference.